For improving the strength, hardness, high temperature resistance, corrosion resistance and wear resistance of the component, the existing technology often plating coatings on metal or other solid materials by chemical or physical method. The coating component has important significance in improving the performance of modern machinery, including automotive, aerospace, and various high temperature wear resistance devices. Evaluation of the mechanical properties of coating must be solved in the engineering application, so as to improve the process and its service life. Residual stress, which is one of the vital parameters of coating, is the stresses existing in the target object and maintaining self-balance, at the condition of zero applied loadings. Both coating and substrate are prepared at high temperature and then cooled to room temperature generally. During this process, due to the mismatch of thermal expansion coefficient between coating and substrate, the smaller thermal expansion coefficient side is under tensile stress, while the larger thermal expansion coefficient side is under compressive stress. This is the main reason for residual stress generated in coating. The existing of residual tensile stress may cause micro cracks in the coating, which reduce the structure performance and have a great influence on its resistance of oxidation, corrosion and high temperature. Therefore, accurately determining the residual stress in coatings has great significance.
At present, the commonly used methods for testing residual stress in coatings mainly include the curvature measurement method based on Stoney formula or the X-ray diffraction method.
For curvature measurement method based on Stoney formula, the principle is that the single-face coated substrate will generate bending deformation under the effect of residual stress. In many cases, the curvature radius can be measured by laser interference instrument or surface contour graph. The residual stress of coating can be calculated by using the Stoney formula according to substrate curvature radius. However, there are some problems in this method: due to the curvature of coated sample is small and it does not equal along the length direction, the curvature is difficult to be measured accurately. In addition, most components are symmetrical coated or in asymmetrical state for which the stiffness of the substrate is much larger than that of the coating, thus no bending deformation could be measured. In fact, Stoney formula is available only for single-face coating and thin substrate sample. Since the residual stress is not a material constant, the measured residual stress in the coating by Stoney formula reflects the stress in the testing piece, not real stress in symmetrical components.
For X-ray diffraction method, its principle is using X-ray diffraction to measure the elastic strain results from the lattice spacing change which is caused by stress. Test samples are often using powder or small blocks, so it only reflects partial performance of sample and it cannot represent the residual stress of the whole component. Meanwhile, the results of the X-ray diffraction method are inaccurate and have large divergence.
It must be emphasized that the residual stress is not a material constant. For the same kind of coating, when the sizes of substrates are different, the residual stress in coating is also different. For the actual service coating, the above mentioned methods are difficult to be used in site.